Some of existing air-conditioning apparatuses include, as in the case of a multi-air-conditioning apparatus for building, a heat source unit located outside a building and indoor units located inside the building (see Patent Literature 1). Refrigerant circulating in a refrigerant circuit of such an air-conditioning apparatus transmits or receives heat to or from air supplied to a heat exchanger of the indoor unit, thereby heating or cooling the air. Then the heated or cooled air is blown into a space to be air-conditioned, to heat or cool the space.
Normally the building includes a plurality of indoor spaces, and hence such an air-conditioning apparatus accordingly includes a plurality of indoor units. In addition, when the building is large the refrigerant pipe connecting between the outdoor unit and the indoor units may even reach 100 meters. When the pipe connecting between the outdoor unit and the indoor units is longer, the amount of the refrigerant to be loaded in the refrigerant circuit increases as much.
The indoor units of the multi-air-conditioning apparatus for building are normally installed and utilized in indoor spaces where people are present (e.g., offices, living rooms, and stores). In case that for some reason the refrigerant leaks out of the indoor unit installed in the indoor space, serious concerns may arise from the viewpoint of impact to human body and safety, because some kind of refrigerants are flammable and/or poisonous. Even when the refrigerant is harmless to human body, the leakage of the refrigerant leads to a decrease in oxygen concentration in the indoor space, which may exert a negative influence to human body.
Accordingly, an air-conditioning apparatus including dual loops has been proposed, in which refrigerant is used in a primary loop and water or brine, which is harmless, is used in a secondary loop, to condition the air in a space where persons are present (see, for example, Patent Literature 2).